It's been noticed that even at recommended doses Excel kills of algae, and is popularly used to kill some species of Algae like Hair Algae at 2-3x the recommended dosage. However this is not with the manufacturer's recommendation.

Some Excel users have noticed a rise in nitrites or nitrates after introduction. Seachem's position is Excel shouldn't affect the nitrogen cycle of an aquarium, but there are online reports of this happening. (Possibly due to the product killing off algae).